This invention relates to an image reconstructing apparatus which is applied to a tomographic apparatus or the like, and more particularly relates to an image reconstructing apparatus which employs X-rays, gamma rays, neutron beams or light beams to irradiate an object under test from a plurality of directions, and obtains internal information of the object in the form of a reconstructed image on the basis of transmission data, namely, projection data in every direction.
A conventional image reconstructing apparatus is as shown in FIG. 7.
In the image reconstructing apparatus, as shown in FIG. 7, a ray source 80 applies, for instance, X-rays to an object 81 under test, and a measuring instrument 82 detects a dose of X-rays passed through the object. The dose of X-rays thus detected is a projection datum which is a result of adding internal information of the object in the direction of irradiation. The projection datum is converted into a digital value by a A/D (analog-to-digital) converter 83, which is provided to a comparator 84. In the image reconstructing apparatus, an image memory 85 is used to store a reconstructed image. The reconstructed image has picture elements the number of which corresponds to the number of internal information of the object. The data f(i,j) of the picture elements are summed in the direction corresponding to the direction of irradiation of the object 81, and provided to the comparator 84. At the beginning, the data f(i,j) of the picture elements have suitable initial values. In the comparator 84, the projection datum is compared with the sum of the data f(i,j) of the picture elements taken in the direction of irradiation, to obtain the difference value therebetween. The difference value is provided to a computer system 86. In the computer system 86, the difference values are subjected to a prescribed digital operation, to provide correction data. The correction data are superposed on the respective data f(i,j) of the elements to correct the data f(i,j) so that the difference between each internal information of the object 81 and the corresponding datum f(i,j) may be reduced.
Thereafter, either the ray source 80 or the object 81 is rotated by a predetermined angle .omega., and on the basis of the projection data which are obtained with the changed direction of irradiation and the stored reconstructed image the above-described operations are carried out again. In this manner, the reconstructed image is sequentially renewed. The image processing operation is carried out until the difference between the reconstructed image and the internal information of the object becomes less than a predetermined threshold difference.
The above-described image reconstructing method, being disclosed in the literature "Iwanami Lectures, Information Science - 21, (Pattern Recognition and Figure Processing)", pp. 192 to 195, published on Mar. 10, 1983, is well known, as a iterative approximation method, in the field of image processing.
In the image reconstructing apparatus according to the conventional iterative approximation method, its image processing operations are electrically carried out; that is, the operation of comparing the projection data with the sum data of the reconstructed image, the operation of processing the difference values resulted from the comparison to obtain the correction data, and the operation of superposing the correction data on the reconstructed image to correct the latter, are digitally carried out.
However, the above-described method of digital processing is disadvantageous in the following points: The accuracy and the resolution of the reconstructed image cannot be improved without increasing the number of picture elements. Hence, each image processing operation, for instance an adding or subtracting operation takes long time. When performing the operations of rotating the reconstructed image and the correction data in response to the change of the direction of irradiation, digitally rotating the picture element matrix is considerably difficult in the points of memory capacity and processing time, thus being unsuitable for realtime processing.